ME 350 – Lecture 11 – Ch 8

1. ME 350 – Lecture 11 – Ch 8 • Polymers are described as plastics and rubbers • Three categories: • Which are plastics? • Which are rubbers?

2. Thermoplastic Polymers - Thermoplastics Solid materials at room temperature but viscous liquids when heated • Can be easily and economically shaped into products • Can be subjected to heating and cooling cycles without significant degradation: • Symbolized by TP

3. Thermosetting Polymers - Thermosets • Cannot tolerate repeated heating cycles • When initially heated, they soften and flow for molding • Elevated temperatures produce: that hardens the material into an infusible solid • If reheated, rather than soften, thermosets: • Symbolized by TS

4. Elastomers (Rubbers) Polymers that exhibit extreme elastic extensibility • Some elastomers can be stretched by a factor of 10 and yet completely recover to their original shape. What would its EL% be? • Although their properties are quite different from thermosets, they share with thermosets a similar:

5. Polymer Notation

6. Polymerization • Addition polymerization • Model of addition (chain) polymerization: (1) initiation, (2) rapid addition of monomers, and (3) resulting long chain polymer molecule with n mers at termination of reaction

7. Degree of Polymerization - n • Since molecules in a given batch of polymerized material vary in length, n for the batch is an: • The mean value of n is called the degree of polymerization (DP) for the batch • DP affects properties of the polymer • Higher DP effects mechanical strength: • Higher DP in the fluid state effects viscosity:

8. Molecular Weight - MW • Molecular weight (MW) of a polymer is the sum of the molecular weights of the mers in the molecule • MW = n times the molecular weight of each repeating unit • Since n, the DP, varies for different molecules in a batch, the molecule weight must be interpreted as:

9. Typical Values of DP and MW

10. Polymer Molecular Structures • Linear structure • Which polymer type(s): • Branched structure • Which polymer type(s):

11. Polymer Molecular Structures • Loosely cross-linked • Which polymer type(s): • Tightly cross-linked • Which polymer type(s):

12. Crystallinity in Polymers • Both amorphous and crystalline structures are possible, although the tendency to crystallize is much less than for metals • For those that can, the degree of crystallinity (the proportion of crystallized material in the mass) is always less than 100%

13. Crystalline Polymer Structure Crystallized regions in a polymer: (a) long molecules forming crystals randomly mixed in with the amorphous material; and (b) folded chain lamella, the typical form of a crystallized region.

14. Crystallinity and Properties • As crystallinity is increased in a polymer • Density: • Stiffness, strength, and toughness: • Heat resistance: • If the polymer is nearly transparent in the amorphous state, when partially crystallized it becomes:

15. Low Density & High Density Polyethylene

16. Strength vs. Temperature

17. Additives • Properties of a polymer can often be beneficially changed by combining it with additives • Additives either alter the molecular structure or add a second phase, in effect transforming the polymer into a composite material

18. Types of Additives by Function Fillers – to strengthen polymer or reduce cost Plasticizers – to soften polymer and improve flow Colorants – pigments or dyes Lubricants – to reduce friction and improve flow Flame retardents – to reduce flammability of polymer Cross‑linking agents – for thermosets and elastomers Ultraviolet light absorbers – to reduce degradation from sunlight Antioxidants – to reduce oxidation damage

19. Elastomer Molecules Model of long elastomer molecules, with low degree of cross‑linking: (a) unstretched, and (b) under tensile stress.

20. Elastic Behavior of Elastomer Molecule • When stretched, the molecules are forced to uncoil and straighten • Natural resistance to uncoiling provides the initial elastic modulus of the aggregate material • Under further strain, the covalent bonds of the cross‑linked molecules begin to play an increasing role in the modulus, and stiffness • With greater cross‑linking, the elastomer becomes and its modulus of elasticity is more

21. Stiffness of Rubber Increase in stiffness as a function of strain for three grades of rubber: natural rubber, vulcanized rubber, and hard rubber.

22. Processing of Polymers • Polymers are nearly always shaped in a • Common operations are • Molding of thermosets is more complicated because of • Thermoplastics are easier to mold and a greater variety of molding operations are available • Rubber processing has a longer history than plastics, and rubber industries are traditionally separated from plastics industry, even though processing is similar

23. Polymers are Replacing other Materials Natural Rubber Nylon (replaces silk) HDPE (replaces cast iron, Cu) PVC Kevlar (replaces metal alloys)

24. Quotes • Never test the depth of the water with both feet. • Silence is often misinterpreted, but never misquoted. • Laugh every day, it's like inner jogging. • There is no key to happiness. The door is always open. • A grudge is a heavy thing to carry. • Have courage and learn from the turtle -- it only makes progress when it sticks out its neck.